feat(music): add Kugou (酷狗音乐) as a music source (#69)

Adds a self-contained Kugou provider (bilibili-style: direct API calls, no
embedded API server, no new npm dependency) plus full backend + WebUI wiring.

Provider (src/music/kugou.ts): search, song-url (with device registration),
lyrics (KRC decode), song detail, playlist, album, personal FM, QR login +
cookie persistence, and quality. Request signing / crypto / KRC decoding are
ported from the MIT-licensed MakcRe/KuGouMusicApi using Node's built-in
crypto and zlib (no third-party crypto packages).

Wiring: the "kugou" platform is threaded through the provider contract, queue,
play-history, bot instance/manager dispatch (getProviderFor + the -k command
flag), index/server composition, the music/player/auth routers (unified
/search/all, /quality, the platform coercions, QR login), the cookie store,
and the WebUI (search source tab + badge, SongCard badge, brand token, and a
Kugou QR/cookie login card in Settings).

Verified live during development: search, lyrics, and album playback resolve
correctly. NOT verifiable in CI (Kugou anti-bot blocks the build host's IP):
play-URL resolution, QR login, and VIP audio — these are built faithfully to
the reference and need end-to-end testing on a non-flagged IP / a Kugou
account. See the header comment in kugou.ts.

Includes src/music/kugou.test.ts (mappers + KRC→LRC). An adversarial review
pass fixed: pagination truncating on filtered counts, an ms/seconds duration
heuristic, dfid soft-fail caching, the /v5/url random-dfid fallback, the FM
body identity, and an unguarded nickname decode.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/api/player.ts

@@ -39,6 +39,7 @@ export function createPlayerRouter(
     if (platform === "bilibili") return "-b";
     if (platform === "qq") return "-q";
     if (platform === "youtube") return "-y";
+    if (platform === "kugou") return "-k";
     return "";
   };
 
@@ -115,7 +116,7 @@ export function createPlayerRouter(
         return;
       }
       const provider = bot.getProviderFor(
-        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local"
+        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local" || platform === "kugou"
           ? platform
           : "netease"
       );
@@ -290,7 +291,7 @@ export function createPlayerRouter(
       // Use the bot's own provider lookup — it already knows about youtube,
       // which the router's constructor params did not.
       const provider = bot.getProviderFor(
-        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local"
+        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local" || platform === "kugou"
           ? platform
           : "netease"
       );
@@ -383,7 +384,7 @@ export function createPlayerRouter(
         return;
       }
       const provider = bot.getProviderFor(
-        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local"
+        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local" || platform === "kugou"
           ? platform
           : "netease"
       );
@@ -614,7 +615,7 @@ export function createPlayerRouter(
         return;
       }
       const provider = bot.getProviderFor(
-        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local"
+        platform === "bilibili" || platform === "qq" || platform === "youtube" || platform === "local" || platform === "kugou"
           ? platform
           : "netease"
       );